Cover crop

A cover crop is a crop that is planted primarily to benefit the soil or growing conditions, rather than as a harvest crop. It may be used to improve the properties of the soil or reduce unhealthy conditions, such as erosion, weeds, and insects. Cover crops are often used in crop rotation to return nitrogen and other nutrients to soil. Properly applied, cover crops can result in economic and ecological benefits for farmers and landowners.

Benefits of planting cover crops include reducing fertilizer costs, cutting use of herbicides and pesticides, increasing soil health to boost yield, preserving soil moisture, reducing erosion, and protecting water quality. The choice of cover crop should be made with one's goals in mind. Some cover crops are more suitable for some situations or growing conditions, such as time of year or climate. Many cover crops may offer additional benefits; for example, they may be harvested. Some may be good forage crops for livestock grazing. Flowering crops, such as clover, may support bees or honey production.

Background

Cover crops have been used in agriculture for centuries. Growing such crops goes hand in hand with crop rotation, which is a much older agricultural practice. Crop rotation is cultivation of food crops in fields in a specific order, so that different crops are planted in each field every year. Ancient civilizations that developed agriculture realized that if they planted the same crops in fields year after year, the soil became depleted of nutrients. Some cultures developed crop rotation systems with the goal of producing high-yield crops quickly, while others focused on maintaining continued production, or long-term use of the fields. It was not until the middle of the nineteenth century that researchers began to understand the principles of successful cropping systems. Rothamsted Research in England established long-term field experiments beginning in the 1840s that continue to operate as of 2018. These experiments include crop rotation. Researchers recorded the benefits of selecting three types of crops: cultivated row, close-growing grains, and rest crops, known as sod-forming crops.

In the colonial United States, many frontier farmers grew the same crops year after year. The soil degraded in about a dozen years, and farmers were no longer able to produce sufficient crops. Many simply moved further west and began anew on untilled land. George Washington realized that simply abandoning farmland and starting fresh elsewhere was unsustainable. He began corresponding with English agriculturist Arthur Young. Young had studied agricultural practices in several European countries. He advocated such progressive practices as crop rotation and the use of machinery to plant seeds at the optimum spacing and depth. As an admirer of Washington, Young responded to his letters with a wealth of knowledge. Washington became an early advocate of crop rotation using cover crops, and he carefully documented his crop rotation system. He classified his plantings as either crops grown to eat and sell or grown to replenish the soil. The latter category included buckwheat, clover, and grass.

During the nineteenth century, farmers regularly planted cover crops, which they called green manure. Among the most popular cover crops in the northern United States were hairy vetch and peas. These added nitrogen to the soil and were often used as forage. In the South, cotton and peanut crops were often rotated with lupines and legume cover crops.

During the 1940s, synthetic fertilizer replaced legume planting in many areas. Herbicides reduced the need for cover crops to suppress weeds. While some farmers continued to plant cover crops, they were often seen as outside the norm in the United States. For the most part, the farmers that practiced the old traditions were organic farmers. By the 1990s, more farmers were phasing out the use of synthetic fertilizers and chemical herbicides. The US Department of Agriculture encouraged sustainable agriculture practices. Recognition of the negative impact of runoff from farmland on bodies of water such as the Chesapeake Bay led to increased promotion and use of cover crops instead of chemicals.

Overview

Cover crops provide multiple benefits. They anchor the soil, preventing erosion, which washes away soil and impacts waterways. They also reduce runoff velocity, or the speed at which water washes off the field. They improve the soil, by adding organic matter, reducing soil compaction, and adding carbon and nitrogen. Cover crops also increase mycorrhizal fungus activity, which helps plants' roots take in water and nutrients. When planted as mulch, cover crops protect the soil from variable temperature changes—for example, keeping the soil cool under the glare of the sun, and reducing the impact of cold temperatures on the soil. This protection may benefit plants in hot regions or when cold weather arrives.

Legumes are among the most common cover crops. This family, which includes beans and peas as well as clovers and vetches, grows in a symbiotic relationship with bacteria in the soil. These bacteria collect nitrogen gas from the air in the soil. They feed the nitrogen to the legume plants, which provide the bacteria with carbohydrates. After the legume crop grows, it can be plowed under in the field, where it provides nitrogen for the next crop, or it may be used in compost, which is used to fertilize other crops. This process is called fixing nitrogen in the soil. Although the bacteria are present in soil, farmers may need to increase the populations in their fields. Before they plant their legume crop seeds, they can coat them with appropriate rhizobacteria to maximize nitrogen fixation.

While some traditional cover crops remain in use, research has found many new options. American farmers have increasingly planted radishes, for example. They offer the benefits of cover crops and also are a cash crop.

Reduced soil erosion is of benefit well beyond the farm. Erosion affects many waterways. For example, the Chesapeake Bay on the East Coast was being affected by nutrients and sediment washed downriver from farmlands. As a result of efforts to educate landowners, cover crop usage in the Chesapeake Bay watershed increased by more than three times from 2005 to 2015. Reduced erosion—and less use of chemicals—have improved the water quality of the Chesapeake Bay. By 2015, levels of sediment and nutrients in the bay had decreased measurably from levels before cover crop use was increased.

Researchers have found global benefits to cover crops. A study published in Agronomy for Sustainable Development in 2017 found that cover crops may decrease the effects of global climate change by keeping carbon in the soil and reducing the amount of sun radiation that is reflected back into the atmosphere. Later studies confirmed the practice, showing that cover cropping trapped significant quantities of greenhouse gases in the soil.

Bibliography

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Kaye, Jason P., and Miguel Quemada. "Using Cover Crops to Mitigate and Adapt to Climate Change. A Review." Agronomy for Sustainable Development, vol. 37, no. 4, 2017.

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